What distinguishes two identical functions?

Question

A source

define two objects with both getter and setter , with the same code

both tests with benchmark.js in node v7.3.0

 const builtInObject1 = (function (object) { let lastA = 1; return Object.defineProperties(object, { a:{ get(){ return lastA }, set(newValue){ lastA = newValue; } } }) })({}); const builtInObject2 = (function (object) { let lastA = 1; return Object.defineProperties(object, { a:{ get(){ return lastA }, set(newValue){ lastA = newValue; } } }) })({}); 

~ add es2015 's getter/setter case

 const builtInObject3 = (function () { let lastA = 1; return { get a(){ return lastA }, set a(value){ lastA = value; } } })(); 

~

 const builtInObject4 = (function (object) { let last = 1; return Object.defineProperties(object, { b:{ get(){ return last }, set(newValue){ last = newValue; } } }) })({}); const builtInObject5 = (function (object) { let last = 1; return Object.defineProperties(object, { c:{ get(){ return last }, set(newValue){ last = newValue; } } }) })({}); 

~

 (new Benchmark.Suite("object-assign-properties")) .add("#built-in object1.a getter and setter", function () { builtInObject1.a = builtInObject1.a + 1; }) .add("#built-in object2.a getter and setter", function () { builtInObject2.a = builtInObject2.a + 1; }) .add("#built-in object3.a es6 getter and setter", function () { builtInObject3.a = builtInObject3.a + 1; }) .add("#built-in object4.b getter and setter", function () { builtInObject4.b = builtInObject4.b + 1; }) .add("#built-in object5.c getter and setter", function () { builtInObject5.c = builtInObject5.c + 1; }) .on('cycle', function(event) { console.log(String(event.target)); }) .on('complete', function() { console.log('Fastest is ' + this.filter('fastest').map('name')); }) .run({ 'async': false }); 

result

 #built-in object1.a getter and setter x 80,459,419 ops/sec ±0.65% (88 runs sampled) #built-in object2.a getter and setter x 3,967,313 ops/sec ±0.36% (91 runs sampled) #built-in object3.a es6 getter and setter x 3,982,725 ops/sec ±0.51% (93 runs sampled) #built-in object4.b getter and setter x 79,608,022 ops/sec ±4.06% (87 runs sampled) #built-in object5.c getter and setter x 78,849,808 ops/sec ±0.82% (92 runs sampled) Fastest is #built-in object1.a getter and setter 

the difference between ops/sec results of these tests makes me confused

Why???

what makes these differences?

accroding to bluebird | Optimization-killers , object3 won't be optimized, but why does object2 get so slow?

link???

• Jsperf tests, it seems both tests run with optimized code in my browser (osx Chrome Canary 57, CPU: i7), just got 3,800,000 opt / sec
  • https://jsperf.com/what-makes-two-same-objects-performance-different/1
  • https://jsperf.com/what-makes-two-same-objects-performance-different/3

related links

• github / bestiejs / benchmark.js | number # 184
• github / nodejs / help | # 442